Ferritic stainless steel, such as SUS430, is economical and has excellent anti-corrosion property, and so has been used in home appliances, kitchen instruments, etc. In recent years, the use of ferritic stainless steel in cooking utensils compatible with induction heating (IH) has been on the increase, as ferritic stainless steel is magnetic. Cooking wares such as pans are often made by bulging or drawing, and sufficient elongation and mean Lankford value (((r-value in the rolling direction)+2×(r-value in the direction of 45° to the rolling direction)+(r-value in the direction orthogonal to the rolling direction))/4, hereafter also referred to as “mean r-value”) are needed in order to form a predetermined shape.
In the case of performing bulging or drawing, it is important that the material of the steel sheet has small anisotropy. For example, in the case of bulging, even when the mean elongation after fracture (((elongation after fracture in the rolling direction)+2×(elongation after fracture in the direction of 45° to the rolling direction)+(elongation after fracture in the direction orthogonal to the rolling direction))/4, hereafter also referred to as “mean El”) of the steel sheet is large, its forming limit is limited by the elongation after fracture in the direction in which the elongation after fracture is smallest in the steel sheet. Accordingly, for stable bulging, the in-plane anisotropy of elongation after fracture (the absolute value of ((elongation after fracture in the rolling direction)−2×(elongation after fracture in the direction of 45° to the rolling direction)+(elongation after fracture in the direction orthogonal to the rolling direction))/2, hereafter also referred to as “|ΔEl|”) needs to be small.
In the case of drawing, earing caused by the in-plane anisotropy of r-value (the absolute value of ((r-value in the rolling direction)−2×(r-value in the direction of 45° to the rolling direction)+(r-value in the direction orthogonal to the rolling direction))/2, hereafter also referred to as “|Δr|”) of the steel sheet occurs. Earing is greater when the steel sheet has larger |Δr|. Accordingly, in the case of drawing the steel sheet having large |Δr|, it is necessary to increase the blank diameter before press forming, which causes a lower production yield rate. Hence, |Δr| needs to be small.
Surface appearance also significantly affects the commercial value of cooking pans and the like. Typically, when forming ferritic stainless steel into a product, surface roughness called ridging appears, degrading the surface appearance of the formed product. In the case where excessive ridging occurs, polishing is required after the formation to remove the roughness, which increases production cost. Ridging therefore needs to be reduced. Ridging derives from an aggregate (hereafter also referred to as “ferrite colony” or “colony”) of ferrite grains having similar crystal orientations. It is believed that a coarse columnar crystalline generated during casting is elongated by hot rolling, and the elongated grains or grain group remains even after hot-rolled sheet annealing, cold rolling, and cold-rolled sheet annealing, thus forming a colony.
In view of the aforementioned problems, for example, JP 4744033 B2 (PTL 1) discloses “a production method for a ferritic stainless steel sheet having excellent workability, comprising: hot rolling a slab of ferritic stainless steel having γmax of 20 or more and less than 70 and then quenching it; coiling the obtained hot rolled sheet at less than 600° C., to obtain a dual phase microstructure of ferrite phase and martensite phase containing a large amount of carbon solid solution; performing intermediate cold rolling at a rolling ratio of 20% to 80% on the dual phase microstructure without hot-rolled sheet annealing, to accumulate a strain in the ferrite phase; then performing long-time annealing (batch annealing) using a box furnace to recrystallize the ferrite phase in which the strain has accumulated and simultaneously recrystallize the martensite phase containing a large amount of carbon solid solution into the ferrite phase to randomize the texture; and further finish cold rolling and recrystallization annealing it to form a ferrite single-phase microstructure”. Here, γmax=420C−11.5Si+7Mn+23Ni−11.5Cr−12Mo+9Cu−49Ti−50Nb−52Al+470N+189. C, Si, Mn, Ni, Cr, Mo, Cu, Ti, Nb, Al, and N denote the contents (mass %) of the respective elements.
JP H9-111354 A (PTL 2) discloses “a production method for an aluminum-containing ferritic stainless steel sheet having excellent ridging resistance and press formability and favorable surface characteristics, comprising: heating, at 1100° C. to 1250° C., a billet having a chemical composition containing, in wt. %, C: 0.02% to 0.05%, Si: 1.0% or less, Mn: 1.5% or less, N: 0.02% to 0.05%, Cr: 15% to 18%, and Al: 0.10% to 0.30%, with a balance being Fe and incidental impurities; then hot rolling the billet and ending the hot rolling at a final pass delivery temperature of 950° C. or more; cooling the hot rolled sheet at a cooling rate of 20° C./s to 80° C./s to a coiling temperature of 500° C. to 650° C. so that the hot rolled sheet is made of multi-phase of ferrite phase and martensite phase and has martensite of 10% to 20% in volume fraction; annealing the obtained hot rolled sheet in a temperature range of 850° C. to 980° C. for 180 seconds to 300 seconds; then performing hot-rolled sheet annealing of quenching the steel sheet at a cooling rate of 15° C./s or more; and further cold rolling and final annealing the hot-rolled and annealed sheet to form a ferrite single-phase microstructure”.